This invention relates to an apparatus for active reduction of noises transmitted into a space from noise sources by producing control sounds for interference with the transmitted noises.
For example, British Patent No. 2,149,614 discloses a conventional active noise reduction apparatus for use in airplane passenger compartments or other closed spaces. The conventional active noise reduction apparatus is applicable to reduce noises transmitted from a single source of noises having a fundamental frequency f.sub.0 and its higher harmonics f.sub.1 to f.sub.n. The noise source is an engine or the like placed in the exterior of the closed space. A plurality of microphones are placed at different positions within the closed space for detecting the sound pressures applied thereon. In order to produce control sounds for interference with the transmitted noises, a plurality of loudspeakers are placed at different positions within the closed space. The loudspeakers are driven by drive signals having frequencies in opposite phase to the frequencies f.sub.0 to f.sub.n of the transmitted noises to cancel the transmitted noises. A "WIDROW LMS" algorithm developed for multiple channels is used to drive the loudspeakers. The "WIDROW LMS" algorithm is described in an article published 1975, in PROCEEDINGS OF THE IEEE. Vol. 63, page 1692, entitled "Adaptive Noise Cancellation: Principles and Applications". The "WIDROW LMS" algorithm developed for multiple channels is described in an article published 1987, in IEEE TRANS. ACOUST., SPEECH, SINGLE PROCESSING, VOL. ASSP-35, PP. 1423-1434 entitled "A MULTIPLE ERROR LMS ALGORITHM AND ITS APPLICATION TO THE ACTIVE CONTROL OF SOUND AND VIBRATION".
The LMS (least mean square) algorithm is one of a number of appropriate algorithms for use in updating the filter coefficients of adaptive digital filters. For example, in a so-called Filtered-X LMS algorithm, all of the transfer function filters modeled on the transfer functions between the loudspeakers and the microphones are set for all of the loudspeaker-microphone combinations. The filter coefficient of each of the variable filter coefficient digital filters provided for the respective loudspeakers is updated in a manner to reduce the value of the performance function calculated based upon the residual noise levels detected by the respective microphones.
The conventional active noise reduction apparatus is designed on such an assumption that the filter accurately represents the acoustic transfer characteristic between the loudspeaker and the microphone. It is, therefore, impossible to reduce the noises if there is a great difference between the acoustic transfer characteristic represents by the filter and the acoustic transfer characteristic of the actual physical space.
Japanese Patent Kokai No. 3-259722 discloses another active noise reduction apparatus applied to a refrigerator. The active noise reduction apparatus employs a microphone to measure the sound pressure at a predetermined position within the refrigerator and a loudspeaker to produce a control sound so as to cancel the noises produced from the compressor used in the refrigerator before the noises are emitted through the duct to the exterior. The control sound is produced based upon the operating condition of the compressor. Each time the compressor comes to a stop, an identification sound is produced to measure an acoustic transfer characteristic between the loudspeaker and the microphones. The measured acoustic transfer characteristic is utilized to identify the filter. However, this conventional active noise reduction apparatus cannot be applied directly to a vehicle passenger compartment since the passenger will be put to great annoyance by the identification sound. In addition, the temperature and humidity change in a short time to a great extent in the vehicle passenger compartment. This causes a deviation of the acoustic transfer characteristic represented by the filter from the actual acoustic transfer characteristic of the actual physical space even though the filter coefficient is updated each time the engine comes to a stop. The deviation will increase with time.